System and method for addressing data in memory

ABSTRACT

A digital signal processor having a CPU with a program counter register and, optionally, an event context stack pointer register for saving and restoring the event handler context when higher priority event preempts a lower priority event handler. The CPU is configured to use a minimized set of addressing modes that includes using the event context stack pointer register and program counter register to compute an address for storing data in memory. The CPU may also eliminate post-decrement, pre-increment and post-decrement addressing and rely only on post-increment addressing.

BACKGROUND

Modern digital signal processors (DSP) faces multiple challenges. Workloads continue to increase, requiring increasing bandwidth. Systems on a chip (SOC) continue to grow in size and complexity. Memory system latency severely impacts certain classes of algorithms. Moreover, modes for addressing data in memory may be complex and/or may not address the needs of many DSPs.

SUMMARY

Examples described herein include a method for addressing data in a memory. The method comprises using an event context stack pointer as a base address. Other examples described herein include a digital signal processor. The digital signal process comprises a CPU. The CPU comprises a program counter register, which the CPU is configured to use as a base address for storing data in memory. The CPU may in addition be configured to address memory using post-increment addressing, and not configured to address memory using pre-increment, pre-decrement or post decrement addressing. The CPU may also comprise an event context stack pointer register for saving and restoring event handler context when higher priority event preempts a lower priority event handler. The CPU may alternatively use the event context stack pointer register as a base address for storing data in memory.

Other examples described herein include a digital signal processor system. The digital signal processor system comprises a memory and a digital signal processor. The digital signal processor comprises a CPU. The CPU comprises a program counter register, which the CPU is configured to use as a base address for storing data in memory. The CPU may in addition be configured to address memory using post-increment addressing, and not configured to address memory using pre-increment, pre-decrement or post decrement addressing. The CPU may also comprise an event context stack pointer register for saving and restoring event handler context when higher priority event preempts a lower priority event handler. The CPU may alternatively use the event context stack pointer register as a base address for storing data in memory.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now be made to the accompanying drawings in which:

FIG. 1 illustrates a DSP according to embodiments described herein; and

FIG. 2 illustrates an exemplary event context stack pointer register.

DETAILED DESCRIPTION

FIG. 1 illustrates a block diagram of DSP 100, which includes vector CPU core 110. As shown in FIG. 1, vector CPU 110 includes instruction fetch unit 141, instruction dispatch unit 142, instruction decode unit 143, and control registers 144. Vector CPU 110 further includes 64-bit register files 150 (for example, designated registers A0 to A15 and D0 to D15) and 64-bit functional units 151 for receiving and processing 64-bit scalar data from level one data cache (L1D) 112. Vector CPU 110 also includes 512-bit register files 160 and 512-bit functional units 161 for receiving and processing 512-bit vector data from level one data cache (L1D) 112 and/or from streaming engine 113. Vector CPU 110 may also include debug unit 171 and interrupt logic unit 172.

DSP 100 also includes streaming engine 113. As described in U.S. Pat. No. 9,606,803 (hereinafter “the '803 patent”), incorporated by reference herein in its entirety, a streaming engine such as streaming engine 113 may increase the available bandwidth to the CPU, reduces the number of cache misses, reduces scalar operations and allows for multi-dimensional memory access. DSP 100 also includes, in the vector CPU 110, streaming address generators 180, 181, 182, 183. As described in more detail in a U.S. Patent Application entitled, “Streaming Address Generation” (hereinafter “the Streaming Address Generation application”), filed concurrently herewith, and incorporated by reference herein, the streaming address generator 180 generates offsets for addressing streaming data. While FIG. 1 shows four streaming address generators, as described in the concurrently filed application, there may one, two, three or four streaming address generators and, in other examples, more than four. As described in the Streaming Address Generation application, offsets generated by streaming address generators 180, 181, 182, 183 are stored in streaming address offset registers SA0 190, SA1 191, SA2 192 and SA3 193, respectively.

The control registers 144 include a program counter (PC) register 121 and, optionally, one or more event context stack pointers (ECSP) 122. The ECSP register 122 is used by the DSP to save and restore event handler context when a higher priority event preempts a lower priority event handler. The ECSP register 122 preserves context in the event of interrupts and contain the address used to stack machine status when an event is detected. FIG. 2 illustrates an exemplary ECSP register, which includes an address 21 and a nested interrupt counter 22.

To write data from the CPU, a store operation is typically used. To read data into the CPU, a load operation is typically used. To indicate the address in memory for reading or writing data, a base address is typically provided as an operand, and optionally an offset.

The base address register can be any of A0-A15 (preferably A0-A15), D0-D15 64-bit scalar registers. Such registers are collectively referred to hereinafter as “baseR.” The program counter (PC) or event context stack pointer (ECSP) control registers can also be used as the base address register.

The base address register may also be a 64-bit register. In this case, because the virtual address size is typically 49-bits, the remaining upper bits (e.g., 15 bits) of this register are not used by the address generation logic or uTLB lookup. However, those bits are checked to make sure they are all 0's or all 1's, if not, an address exception may be generated.

A constant offset can be a scaled 5-bit unsigned constant or a non-scaled signed 32-bit constant. This value may be scaled by the data type, e.g., element size (shift by 0, 1, 2, or 3 if the element size is Byte, Half-Word, Word or Double-Word respectively) and the result (up to 35-bits result after the shift) may be sign-extended the rest of the way to 49 bits. This offset may then added to the base address register. The offset value may default to 0 when no bracketed register or constant is specified.

Load and store instructions may use for the offset, for example, the A0-A15 (preferably A8-A15), D0-D15 registers. ADDA/SUBA instructions, which perform linear scaled address calculation by adding a base address operand with a shifted offset value operand, can use, for example, the A0-A15, D0-D15 registers. Collectively, the valid register offset is denoted as offsetR32. The streaming address offset registers SA0 190, SA1 191, SA2 192 and SA3 193 may also be used for the offset for load, store, ADDA or SUBA instructions. Streaming address offset registers SA0 190, SA1 191, SA2 192 and SA3 193 can be used with or without advancement. Exemplary syntax used for advancement of streaming address offset register SA0 is “SA0++”, which will advance the offset after it is used to the next offset, as described in the Streaming Address Generation application.

Postincrement addressing updates the base register by a specified amount after address calculation. For postincrement addressing, the value of the base register before the addition is the address to be accessed from memory. Post increment operation of the base address register is denoted as baseR++. Such an operation will increment the base register by one addressed element size after the value is accessed.

Using the program counter 121 as the base address may be referred to as PC-relative addressing mode. PC-relative references may be relative to the PC of the fetch packet containing the reference. This may be true for an execute packet spanning fetch packet as well, where the PC reference may be relative to the fetch packet which contains the instruction that has the PC-relative addressing mode. Specifically, the address that is used for the base address when using the PC relative addressing mode is, in at least one example, the address of the fetch packet containing the .D unit instruction that has the PC relative addressing mode.

For example, for this code sequence:

LDW .D1*PC[0×30], A0

∥ LDW .D2*PC[0×34], A1

If the instruction on .D1 and the instruction on .D2 are in different fetch packets due to spanning, then they will end up using different values for the PC.

Using an ECSP 122, shown in FIG. 2, as the base address provides the benefit that the address will be preserved in the event of interrupts, or even nested interrupts.

Table 1 shows addressing modes for load and store operations according to examples described above:

No modification of base address Addressing type register Post-increment Register indirect, baseR[0] baseR++[0] no offset base register incremented by element size Register indirect, baseR baseR++ offset by access base register size in bytes incremented by element size Register relative baseR[ucst5] baseR++[ucst5] with 5-bit unsigned base register constant offset, scaled incremented by ucst5 constant Register relative baseR(scst32) baseR++(scst32) with 32-bit signed base register constant offset, unscaled incremented by scst32 constant Register relative baseR[offsetR32] baseR++[offsetR32] with 32-bit register base register index, scaled incremented by offset Register relative baseR[SA] baseR[SA++] with Streaming Address advances SA by Generator as register one iteration, index, scaled base register unchanged Program Counter PC[ucst5] Not Supported Register relative with 5-bit unsigned constant offset, scaled Program Counter PC(scst32) Not Supported Register relative with 32-bit signed constant offset, unscaled Event Context Stack ECSP[ucst5] Not Supported Pointer Register relative with 5-bit unsigned constant offset, scaled Event Context Stack ECSP(scst32) Not Supported Pointer Register relative with 32-bit signed constant offset, unscaled

By limiting the addressing modes to only a few options and by including only post-increment (i.e., no pre-increment or pre-/post-decrement), design verification spaces are reduced, and fewer opcode bits are needed.

Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims. 

What is claimed is:
 1. A method for addressing data in a memory, the method comprising: maintaining a program counter register; maintaining an event context stack pointer register that is separate from the program counter register and that stores an address and a nest count associated with a set of nested interrupts, wherein an offset is based on the nest count; receiving an event interrupt; in response to the event interrupt, using the event context stack pointer register to save and restore event handler context; receiving an instruction that specifies a memory address of the memory based on the address in the event context stack pointer register; in response to the instruction, using the event context stack pointer register as a base address to access the memory; and offsetting the address stored in the event context stack pointer register with the offset to determine the memory address, wherein the offset is selectively scaled based on a data type.
 2. The method of claim 1, wherein: the offset is an unsigned 5-bit constant; and the offset is scaled.
 3. The method of claim 1, wherein: the offset is a 32-bit constant; and the offset is not scaled.
 4. A digital signal processor comprising: a central processing unit (CPU) configured to couple to a memory, wherein: the CPU includes a program counter register and an event context stack pointer register for saving and restoring event handler context when a higher priority event preempts a lower priority event handler; and the CPU is configured to: receive an instruction that specifies a memory location in the memory based on the event context stack pointer register that stores an address and a nest count associated with a set of nested interrupts, wherein an offset is based on the nest count, and wherein the offset is selectively scaled based on a data type; use the address and the offset of the event context stack pointer register as a base address to determine the memory location; and in response to the instruction, load or store data at the memory location in the memory.
 5. The digital signal processor of claim 4, wherein: the offset is an unsigned 5-bit constant; and the offset is scaled.
 6. The digital signal processor of claim 4, wherein: the offset is a 32-bit constant; and the offset is not scaled.
 7. A digital signal processor system comprising: a memory; a digital signal processor coupled to the memory comprising: a central processing unit (CPU) comprising a program counter register and an event context stack pointer register for saving and restoring event handler context when higher priority event preempts a lower priority event handler, and wherein the event context stack point register stores an address and a nest count associated with a set of nested interrupts, wherein an offset is based on the nest count, wherein the offset is selectively scaled based on a data type, and wherein the CPU is configured to use the address and the offset of the event context stack pointer register as a base address for loading or storing data in the memory at a memory address.
 8. The digital signal processor system of claim 7, wherein: the offset is an unsigned 5-bit constant; and the offset is scaled.
 9. The digital signal processor system of claim 7, wherein: the offset is a 32-bit constant; and the offset is not scaled.
 10. The method of claim 1 further comprising, in response to the instruction, incrementing the address stored in the event context stack pointer register after the using of the event context stack pointer register to access the memory.
 11. The method of claim 10, wherein the instruction specifies to increment the address by an addressed element size after the using of the event context stack pointer register to access the memory.
 12. The method of claim 10, wherein the instruction specifies to increment the address by a constant after the using of the event context stack pointer register to access the memory.
 13. The digital signal processor of claim 4, wherein the instruction specifies a post-increment operation of the value of the event context stack pointer register.
 14. The digital signal processor of claim 13, wherein the instruction specifies the post-increment operation to increment the value of the event context stack pointer register by an addressed element size.
 15. The digital signal processor of claim 13, wherein the instruction specifies a constant and specifies the post-increment operation to increment the value of the event context stack pointer register by the constant. 